When it rains here on Earth, the raindrops are made of water. When it rains on Saturn’s moon Titan, the raindrops are made of methane. And on the objects known as brown dwarfs, the raindrops could even be made of iron.

Brown dwarfs are often described as “failed stars.” They formed in the same way as stars -- from collapsing clouds of gas and dust. But they didn’t grow massive enough to produce energy through the nuclear reactions that power true stars. Instead, they shine mainly because their gravity squeezes them and heats them up.

They’re much cooler than stars, though, which creates some interesting differences. In particular, brown dwarfs appear to be topped by clouds -- perhaps like the bands of clouds that encircle the giant planet Jupiter.

Depending on the brown dwarf’s temperature, age, and other factors, the clouds can be made of anything from water vapor to vapors of minerals and iron. As the brown dwarfs age they get cooler, so material in the clouds condenses and falls toward the surface -- perhaps even droplets of liquid iron.

This topping of clouds may conceal giant storms that swirl through the brown dwarf’s upper layers of hot gas. On an older brown dwarf, these storms may blow away the clouds completely, letting the hot gas shine through -- making the old brown dwarf shine hotter and brighter than its younger kin.